The present invention relates generally to semiconductor integrated circuit (IC) devices, and more particularly to a semiconductor structure for preventing damage on IC devices during a fuse blowing process.
The steady down-scaling of complementary metal-dielectric-semiconductor (CMOS) device dimensions has been the main stimulus to the growth of microelectronics and the computer industry over the past two decades. The more an IC is scaled, the higher becomes its packing density. Today, after many generations of scaling, the smallest feature in a CMOS transistor is approaching nano-scale dimensions. As a result of the increased packing density, the complexity of ICs has dramatically increased. This increase in IC complexity leads to a corresponding increase in design and fabrication errors during the development and manufacture of ICs. It is desired to modify a portion of the functionality of an IC without starting a new costly IC development effort.
Fuses are routinely used in the design of ICs, and in particular in memory devices as elements for altering the circuit configuration for those memory devices. As such, memories are commonly built with programmed capabilities wherein fuses are selectively “blown” (melted away) by, as an example, a laser beam. Fuse elements are typically made of materials, such as aluminum, copper, polysilicon, silicide, and other conductive metal or alloy.
It is well known that random access memories (RAM) are designed with redundancies which include spare columns or rows of electric elements. When any of the elements fails, the defective rows and columns are replaced by the corresponding spare elements. Fuses, which are strategically placed throughout the IC, accomplish disabling and enabling of these spare elements.
The use of a laser beam to “blow” the fuses to modify the circuit configuration of an IC can induce certain failure mechanisms. Fuses are usually fabricated on the top metal layer of an IC for easy laser access. A laser beam is directed onto the desired fuse to melt the copper (or other materials) until an open occurs to obtain a desired circuit modification. However, only a small percentage (˜30%) of the laser energy is actually directed onto the fuse. Significant laser energy (˜70%) penetrates subsequent lower layers (typically comprised of dielectric insulating layers) down to the semiconductor substrate. As a result, significant damage can easily occur in areas other than those occupied by the fuses.
One failure mechanism that occurs due to the laser blow process is the damage to the substrate below the fuse due to the excess laser energy. In conventional designs, no electronic devices or circuits are placed beneath the fuse due to potential damage during the laser blow process. This results in unused areas of the substrate, which decreases packaging densities. A conventional method to eliminate this failure mode is to incorporate a reflective protective surface structure on the layer beneath the fuse. This reflective structure protects subsequent layers and the substrate from laser damage. However, this reflective structure is not an ideal solution to protect the IC from laser induced cracks, low K dielectric thermal shrinkage, or laser beam burn out.
Another failure mechanism that occurs during the process of blowing a fuse with a laser is that the gate dielectric layer of a device close to the fuse can be irreparably damaged by laser energy. One conventional method to reduce this gate dielectric layer damage is to utilize thick gate dielectric layers. However, this is not a practical solution for submicron geometry ICs due to size and performance limitations. Another conventional method to alleviate this condition is to add a protection diode either in series or parallel with the fuse. The protection diode dissipates excess energy before it is applied to the gate dielectric layer of a device close to the fuse.
Therefore, desirable in the art of laser fuse blowing are alternative designs that increase the effective layout area utility rate while avoiding failures induced by the fuse blowing process.